I am using TFS 2012 to develop a application that will be available as a website and a mobile application, i.e. Windows Phone, Android, etc. While I've been building up a list of features for this application I've noticed that a lot of them will be available across all platforms and I'm not to sure how to manage them within a product backlog.

For example, there will be an option to sign in with a Facebook account and user will be able to do this on website and mobile applications.

So my though was I would create a product backlog item "Sign in with Facebook account" and assign it to an area called "Website". I would then create another backlog item, with the same title, but this time assign it to an area called "Windows Phone". Therefore my backlog would have two items, both with the same title, but different areas.

The idea is I could assign the "Sign in..." backlog item for the website to one sprint and then assign the "Sign in..." backlog item for Windows Phone to another sprint.

Seeing as I'm new using Agile/Scrum would this be considered a viable way of managing a product backlog?

Break them down as you find it useful. If you feel comfortable forecasting pulling an idea/need/requirement into an iteration without breaking it down across platforms. That's up to you... there is no correct way per-se.
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Ryan CromwellNov 13 '12 at 3:53